The filter is densest at the edge and tapers to clear by the middle, with a medium-edged line of transition between the dense and clear areas. A neutral density filter with a medium-edged line of transition can filter the sky while minimizing its effect on the land. It is particularly useful for images that include elements like rooftops and mountains that protrude into the sky.
Neutral density filters do not affect the coloration of the image and this filter provides reduction in light from entering a lens.